fix: translate the tab bar and label the unified inbox and drawer buttons

The bottom tabs showed their route names ("Mail", "Calendar", …) in
every language; they now use the webmail's navigation names, and an
unavailable tab says so with one translated message instead of an
English "(unavailable)" suffix. The first loading screen is
translated as well.

The Unified Inbox selection toolbar, its back and clear-search buttons,
and the drawer's close, expand/collapse and remove-account buttons get
screen reader labels. Its unknown-sender fallback, selection count and
"account(s) could not be loaded" banner are proper translations and
plurals now.

The key harvest and the translation coverage test did not look at
App.tsx, which sits next to src/; they do now.

scripts/harvest-rn-keys.mjs

@@ -68,7 +68,8 @@ const KEY_ONLY_RE = /\btr?\(\s*'([^']+)'/g;
 
 const added = [];
 const missingNoFallback = [];
-for (const file of walk(SRC)) {
+// App.tsx (the tab bar and loading screen) sits next to src/.
+for (const file of [...walk(SRC), join(ROOT, 'App.tsx')]) {
   const text = readFileSync(file, 'utf8');
   const withFallback = new Map();
   for (const m of text.matchAll(CALL_RE)) {